1. Field of the Invention
The present invention relates to a method for fusing and connecting so-called hermetic optical fibers which have a thin film coating layer of, for example, a metal or carbon on a surface of an optical fiber glass.
2. Description of Related Art
Hitherto, a method for fusing and connecting optical fibers by an aerial discharge heat is used to permanently connect the optical fibers. The reasons therefor are that (1) a connection loss is small, (2) excellent reliability for a long term is obtained because the connected part is reinforced with a heat-shrink tube and the like after the connection.
A hermetic optical fiber has (1) improved water resistance and (2) improved reliability for a long term by coating a thin film of a metal, carbon or the like on an optical fiber surface. Since the surface of the hermetic optical fiber is electrically conductive, the strength after the connection is decreased. The reason therefor has been thought that the hermetic layer material is incorporated in the fused and connected part of the optical fiber.
In order to solve this problem, we proposed a method for connecting carbon hermetic optical fibers, which comprises removing a carbon layer immediately before setting the fibers on a fusing instrument, and fusing and connecting the hermetic optical fibers through an aerial discharge (Japanese Patent Kokai Publication No. 195304/1990 (Japanese Patent Application No. 14638/1989)).
A sample connected by this method and a sample fused and connected without removal of the hermetic layer were subjected to a break strength test. The results of the break strength are shown in Table and FIG. 3. The sample prepared by removing the carbon layer and fusing and connecting the optical fibers has the average break strength of almost twice that of the sample without removing the carbon layer. Characteristically, the breakage always occurred on a V-shaped groove made of a metal.
TABLE __________________________________________________________________________ Break number number of Average Maximum Minimum at V No. Steps samples value value value groove __________________________________________________________________________ 1 coverage removal .fwdarw. 20 0.7 1.1 0.5 16 cutting .fwdarw. fusion 2 coverage removal .fwdarw. 20 1.4 1.8 0.8 8 cutting .fwdarw. carbon removal .fwdarw. fusion __________________________________________________________________________
As stated above, the method for fusing and connecting the optical fibers after removing the carbon layer gives a connection having a slight decrease of strength. But, the removal of the carbon layer needs to remove the carbon layer through a thermal decomposition by heating the carbon layer to higher than 500.degree. C., preferably 500.degree. to 1,000.degree. C. thereby requiring a labor and a time.